The electrostatically aided atomization and coating of articles with coating materials such as atomized liquid droplets and pulverulent coating resins, or powders, is well known. There are, for example, the systems and methods illustrated and described in U.S. Pat. Nos. 3,851,618; 3,875,892; 3,894,272; 4,075,677; 4,187,527; 4,324,812; 4,481,557; 4,485,427; 4,745,520; and, 5,159,544, to identify but a few. This listing is not intended as a representation, nor should any such representation be inferred, that a complete search of all relevant art has been conducted or that no better art references than those listed here are available.
Most systems and methods for the charging and application of coating materials by electrostatically aided atomization and coating, and the control of such systems, are designed with the underlying assumption that the coating material being dispensed behaves, to a first approximation, in a resistive manner. For example, the power supplies which provide the potentials from which the coating materials are charged are designed to charge and dispense materials which are electrically charged, but from which the charge migrates moderately quickly through the coating film being deposited on the article toward the grounded article itself. The resistance may be relatively low, as in the case of, for example, water base coating liquids, or relatively high, as in the case of colored coating resins, but the materials do behave in a basically resistive manner. These assumptions concerning the nature of these various coating materials have resulted in the design of the coating material charging power supplies with certain characteristics, such as, for example, relatively constant output voltage.
At some point, however, on the scale of resistivity of these coating materials, applicants have discovered that coating materials cross over from being resistive to a first approximation to being dielectric or capacitive to a first approximation. This phenomenon occurs at least by the time one considers the resistivities and other electrical parameters of some clear powder coatings. There is presently interest in the electrostatically aided application of such powder coatings in the manufacture of automotive vehicles. It has been determined that the unfused coatings can experience back ionization or back emissions from the article to which coating is being applied, resulting in the creation of imperfections, such as "starring" or pinholes and the like, in the applied coating. In many cases, these imperfections are not cured by passage of the coating through, for example, an infrared fusing oven. Consequently, the imperfections remain in the fused coating as, for example, sites where oxidation of the underlying surface of the article can occur. The automotive finish applicator must have a class A quality finish on the clear topcoat layer of an automotive finish. This layer generally must be substantially perfect when it is applied. It is more difficult to repair defects in the topcoat than, for example, the primer coat. Back ionization, if it is permitted to occur, generally causes heavy "orange peel" on a powder coated article after the powder coating is cured.
One explanation for this phenomenon is that the electric charges imparted to the particles of coating material as they are dispensed toward the target are not mobile on the surfaces of the coating material particles because of the relatively higher resistivities of the resins from which clear powder coatings are made. As a result, electrical charge does not migrate from newly deposited particles through underlying, previously deposited particles toward the surface of the article to be coated. Substantially the full charge remains on the deposited particles. Some results of this accumulation of charge are: that the deposited particles basically repel each other; that dispensed particles that have not yet reached the article to be coated are repelled by the relatively static electrical charge previously built up on the surface of it along with the previously applied resin powder; and, that the deposited particles are attracted toward other grounded targets, including exposed, grounded parts of the dispensing equipment. Additionally, free ions that are created by the charging mechanism of the powder applicator cannot migrate through the dry powder film very quickly due to the dielectric properties of clearcoat powders. As a result, electric fields in the range of, for example, 3.times.10.sup.6 volts/meter are produced between the outer layer of the dry powder film and the grounded article being coated by the powder. When such fields accumulate, electrical arcing occurs through the deposited powder coating. This arcing results in surface defects. By controlling the formation of these free ions with a constant current power supply, these surface defects can be greatly reduced. Whatever the actual cause(s) of the phenomenon, the upshot of it is that power supplies which were designed based upon the assumption that coating materials were to a first approximation primarily resistive in nature are less than ideal power supplies for charging and applying coating materials which appear not to be primarily resistive in nature.